Systems are well-known in the art, for use in conjunction with actuator mechanisms, for controlling movement of a valve stem, for example, in response to an input signal or condition. Such systems are usually pneumatic, using compressed air to effect both the control functions and movement of the valve stem. It is common to use a pneumatic relay for a single acting actuator mechanism or a pair of pneumatic relays for a double acting actuator mechanisms. The actuator includes a piston which in turn is coupled to the control device, usually a valve stem, for movement thereof in accordance with the position of the piston. A feedback of information about the position of the controlled device is made to the positioner to verify that the position determined by the input signal or condition has been reached.
Generally, the input signal is converted to a force and applied to a summing means, sometimes referred to as a summing beam or balance beam, which is movable to modulate a nozzle pressure that is used as a command signal for the pneumatic relays. The information as to the position of the controlled device is also converted to a force and applied to the summing means. The summing means is statically balanced by a zero adjustment force and, when the positioner is in equilibrium, the relays are closed and the system is at rest. In practice, the relays usually allow line pressure to act on both sides of the actuator piston, even during equilibrium. The operation of such positioner-actuator systems is well known in the art. Needless to say there is a myriad of design parameters to be considered and many compromises are required because of the need to satisfy a very wide range of actuators and controlled devices.
For example, it is well known that controlling the valve stem position of a small volume actuator mechanism presents an entirely different set of problems than controlling the valve stem position of a large volume actuator mechanism by means of a correspondingly larger actuator mechanism. That is, typical small volume actuators exhibit greater underdamped stem response than large volume actuators due to the smaller volumes which must be changed to effect positioning. When one considers the attributes the positioner must have, such as static positioning accuracy, response time, dynamic stability, and adaptability for the user-in conjunction with cost constraints-the need for positioners capable of good performance over a wide range of operating conditions is readily perceived.
There are many feedback arrangements for indicating the position of the movable element in the controlled device to the summing beam. This type feedback is often referred to as major loop feedback and is sufficient for a great number of designs operating with single-acting diaphragm actuators. However, major loop feedback is insufficient for double-acting actuators having air pressure applied to both sides of a piston which is moved by the pressure differential. Such actuators have required additional feedback, sometimes referred to as minor loop feedback in the positioner to prevent overtravel of the actuator piston yet provide reasonable "crisp" response, that is good response time. The minor loop feeback is negative and may be considered a damping means.
Obviously, if a positioner is designed for use with a particular actuator and controlled device the system can be critically damped and not require any additional feedback or damping. Thus a single-use positioner-actuator system can readily be "tailored" for optimum performance. Realistically, this is an unrealized situation as the cost is prohibitive. Faced with this dilemma, designers make the units to perform adequately based upon "worst case" conditions. In the positioner art that means for use with a small actuator. The low volume of the actuator mechanism makes it susceptible to "overshooting" and going into an oscillatory mode should a reasonably fast response time be demanded. Use of such a positioner with a "heavier" actuator involves substantial performance degradation. Consequently there are many drawbacks to systems presently available because of these cost-performance tradeoffs.
U.S. Pat. No. 3,565,391 discloses a pneumatic valve positioner having minor loop feedback for providing dynamic stabilization between an air relay and a balance beam (summing beam). The valve positioner conventionally operates to command a part (controlled device) to undertake a mechanical motion or excursion in response to an input signal in the form of a gas supplied at a certain signal pressure. An output signal is sent in response to the actual pressure of the received signal to cause the desired motion to occur. The controlled device, in moving, mechanically relocates a reference point which feeds back a signal confirming that the motion has indeed occurred. (This is the major feedback referred to above). The command signal is conventionally generated by a nozzle flapper arrangement which is responsive to the input signal, the output signal, and the feedback signal. The patent attempts to solve the problem of fast response time without overshoot while retaining resolution sensitivity and stability by using a three point balance beam, with one point being controlled by the input signal, another by the output signal, and a third responsive to a feedback signal from the controlled device. The patent thus discloses a conventional summing beam with input signal and a major loop feedback signal indicative of controlled device position and a fixed minor loop feedback ratio, or gain, from the output relay back to the summing beam. The minor loop feedback has a damping effect on the summing beam and the three point system is said to achieve dynamic stability.
The patented system and other prior art systems satisfy definite needs in the art. The system of the invention however, as will be seen, goes well beyond prior art systems and provides the user with a positioner-system that is field-adjustable for optimizing performance with a wide range of actuator types. With the system of the invention the problem of prior art systems because of "worst case" design criteria using a small actuator and actual field conditions using substantially larger actuators, is effectively overcome. With a single system the user may "dynamically tune" the positioner of the invention to produce a "crisp" response without overshoot. Finally, this improvement in performance is accomplished in a very economical and cost-effective manner.